MRAM (magnetic random access memory) architecture can be based on MTJs that sit at the intersection points of orthogonal wires running above and below them. A typical MTJ film stack is shown in FIG. 1. Seen at the top of the MTJ cell stack is cap layer 11, of (for example) tantalum. The reading of stored data can be accomplished by passing a low current through the MTJ. The orientation of its magnetic moment, parallel or antiparallel, determines in which of two possible resistance states tunnel barrier 13 happens to be, thereby defining a pair of binary memory states.
Also seen in FIG. 1 are seed layer 16, pinning layer 15, pinned layer (or layers if a trilayer synthetic antiferromagnetic is used) 14, dielectric tunneling layer 13, and free layer 12. The latter is itself often made up of two or more layers.
To write data, electric current needs to pass through the wires that are closest to the magnetic cells. The magnetic field induced by this current can alter the direction of the magnetic moment. These two sets of orthogonal wires are called word lines and bit lines. Currently, researchers worldwide are attempting to develop the processes needed for successful commercial production of MRAM.
CMP (chemical mechanical polishing) is one of the processes used in the manufacture MRAM devices. A dielectric layer is deposited on the patterned MTJ cells, and then CMP is employed to planarize the topography and remove excess dielectric material over the MTJ stacks thereby leaving the cells inlaid within the dielectric layer.
One of the major problems associated with CMP is the difference in polishing rate between the hard dielectric layer and the relatively soft MTJ cap material. Note that the use of words such as ‘hard’ and ‘soft’ may be somewhat confusing for understanding the protrusion of a soft MTJ from a hard dielectric after polishing. Although it is true that dielectrics are generally harder than metals in terms of their mechanical properties, CMP is a combined chemical and mechanical process that can polish ‘hard’ material faster than ‘soft’ material by selecting an appropriate chemistry, and vice versa.
Ideally, CMP should terminate right at the MTJ cap layer but, due to CMP non-uniformity and pattern dependence, the process requires over-polishing in order to ensure the complete exposure of the MTJ cap layer. If the cap layer removal rate is slower than the dielectric removal rate, MTJ caps will protrude through the dielectric layer at the conclusion of CMP. A protruded MTJ cap is vulnerable to the occurrence of electrical shorts.
Thus, controlling the variation of the cap layer CMP removal and MTJ cap protrusion presents one of the main challenges for MRAM manufacturing processes. However, if protrusion of the cap at the conclusion of CMP cannot be effectively prevented, an alternative way to prevent shorting to the protruding cap needs to be found. The present invention teaches several structures, and methods for their manufacture, that solve this problem.
A routine search of the prior art was performed with the following references of interest being found:
U.S. Pat. No. 6,555,858 (Jones et al) discloses magnetic cladding sidewall spacers.
U.S. Pat. No. 6,475,857 (Kim et al) describes dielectric sidewall spacers, but they are removed. U.S. Patent Application 2004/0205958 (Grynkewich et al) discloses sidewall spacers and a masking tab to reduce the likelihood of electrical shorting by preventing deposition of metallic particles during etching of the metal layers of the MTJ. The spacers also prevent shorting if a via overlying the MTJ stack is misaligned. U.S. Patent Application 2004/0191928 (Shi) shows sidewall spacers on an MTJ stack.